1. Field of the Invention
The present invention relates to an optical disc recording/reproduction device for optical disc recording/reproducing for recording/reproducing information onto/from an optical disc and in particular, to an optical disc recording/reproducing device capable of surely executing focus offset adjustment and spherical aberration correction adjustment regardless of the state of an optical disc mounted.
2. Description of the Related Art
For example, an optical disc recording/reproduction device for recording and/or reproducing data onto/from an optical disc as a disc-shaped optical information recording medium such as a CD and DVD is a data recording/reproduction device characterized in that it can access data at a high speed in non-contact way, with large capacity, and at low cost. By these characteristics, for example, it is widely used as a recording/reproduction device of recording/reproduction device of digital audio data and digital moving picture data or a computer external storage device.
Moreover, recently, the data amount which can be handled by an optical disc such as a CD and DVD has been increased and their density is increased. It should be noted that the data recording density of the optical disc is limited by a device for recording/reproducing information onto/from a disc and especially whether light collection is enabled onto the information recording surface of the optical disc and the size of the optical spot. The size of the optical spot is proportional to the wavelength of the optical flux and inversely proportional to the NA (numerical aperture). Accordingly, in order to increase the density of the optical disc, it is necessary to shorten the wavelength of the light flux generating the optical spot and increase the NA of an objective lens.
In general, when the wavelength of the light flux is shortened and the NA of the objective lens is increased, the affect of aberration is increased. When an aberration is generated in the optical system for generating an optical spot, the optical spot obtained is too large. Especially, the spherical aberration SA is expressed by the following expression:
                    SA        ∝                              Δ            ⁢                                                  ⁢                          T              ·                              NA                4                                              λ                                    (        Expression        )            wherein “ΔT” is an error of the optical disc substrate thickness (hereinafter, referred to “substrate thickness error) and “λ” is a wavelength of the optical flux.
That is, the expression above shows that the spherical aberration is increased in proportion to power 4 of NA of the objective lens and the disc substrate thickness error ΔT, and inverse proportion to the wavelength λ of the optical flux.
Moreover, in an optical disc having an information recording layer formed by two or more layers, a spherical aberration is generated by the interval error between the information recording layers in the same way as the aforementioned substrate thickness error. Accordingly, in the optical disc having a multi-layer structure, it is also necessary to correct the spherical aberration generated by the interval error between a plurality of recording layers.
Conventionally, as an optical pickup configuration capable of correcting such a spherical aberration, for example, there are known a configuration having a beam expander for adding a predetermined spherical aberration into the optical path of optical flux, a configuration having a liquid crystal element for giving a predetermined spherical aberration, or a configuration for giving spherical aberration by changing the interval of two objective lenses in pair in the focusing direction. Moreover, JP-A-2000-011388 and JP-A-2001-222838 disclose a technique to compensate the spherical aberration generated due to the substrate thickness error and the interval error between the information recording layers by using the optical pickup having the aforementioned configuration to intentionally generate spherical aberration.